1. The Field of the Invention
The present invention relates to flowable concrete mixtures and cured concrete obtained therefrom. More particularly, the present invention relates to concrete mixtures that have an aqueous based foam admixture that includes a viscosity modifier.
2. Related Technology
Concrete mixtures are composite materials that are usually composed of water, cement, and aggregate. Common aggregates include sand, gravel, or crushed stone. Concrete is a well-known structural component with typical compressive strengths of about 2500 psi, when cured.
Admixtures are often added to concrete to give the concrete mixture and or the cured concrete desired properties. For example, admixtures can be used to lower the concrete's density, improve the concrete's workability, improve the concrete's strength to weight ratio, give the concrete insulating properties, and/or enhance the acoustic properties of the concrete, among others. These beneficial properties are often accomplished by adding several different admixtures.
Viscosity modifying agents (VMA), also known as viscosity modifiers, Theological modifiers, and rheology modifying agents, can be added to the concrete mixture to facilitate uniform flow of the particles and reduces bleed, or free water formation. Viscosity modifiers are often water-soluble polymers and function by increasing the apparent viscosity of the mix water.
Viscosity modifiers are particularly useful for concrete mixtures that are highly flowable. Water and water reducers are added to concrete mixtures to make the mixture flowable. However, there is a limit to the amount of water and water reducers that can be added to the concrete mixture before the components (e.g. aggregate) begin to separate. Viscosity modifiers are flowable but viscous. The viscosity modifiers help to suspend aggregates and other components in the mixture, thereby holding the mixture together.
Despite the beneficial properties of viscosity modifiers, viscosity modifiers are not widely used in concrete because the amount of viscosity modifier that must be added to see beneficial results is often cost prohibitive. Currently, the use of viscosity modifiers is mostly limited to high performance concrete mixes such as so called self-compacting concrete (SCC mixtures).
The flowability and consistency of SCC mixtures is such that the mixture will consolidate or compact without the need to vibrate the concrete after it has been placed. Consolidating the concrete to remove air bubbles is important to maintain the integrity and strength of the cured concrete. Since SCC mixtures self compact, these mixtures require less manpower and avoid human error associated with manually vibrating the placed concrete. Unfortunately, the cost of making SCC mixtures often outweighs the benefits of SCC mixtures. The viscosity modifier is a significant cost in making SCC concrete. The cost of the viscosity modifier is in large part a consequence of the amount of viscosity modifier that has to be added to obtain the desired results. For example, SCC mixtures often include 15-20 oz of viscosity modifier per 100 lbs cement to achieve the desired rheological effect.